1. Field of the Invention
The present invention relates to a sheet-conveying device for conveying a sheet material, such as for example a sheet of paper, an envelope, or a postcard, to an image-forming apparatus, such as a printer and a copy machine.
2. Description of the Related Art
In general, many sheet-conveying devices have a skew correction function. When a sheet is skewed, the sheet is conveyed at an angle with respect to a conveying direction of the sheet due to insufficient paper feed accuracy of a sheet-conveying device. In image-forming apparatuses, the skew of the sheet causes paper jams or degradation of print quality. FIG. 7 shows a typical known structure used for skew correction.
Referring to FIG. 7, the structure includes a conveying roller 502 that tries to convey a sheet 505 in a conveying direction 50A and a roller 503 that is disposed so as to face the conveying roller 502 and that is inclined with respect to the conveying direction 50A. The inclined roller 503 and the conveying roller 502 convey the sheet 505 while nipping the sheet 505 therebetween. A reference surface 501 is provided on a side of a conveying path. The reference surface 501 is parallel to the conveying direction 50A and is perpendicular to a plane along which the sheet 505 is conveyed. A member including the reference surface 501 has a shape like a wall or a projection so that an edge of the sheet 505 can come into contact with the reference surface 501. Accordingly, the member including the reference surface 501 serves as a guide that defines a reference position in the width direction of the conveying path and that guides the sheet 505 in the conveying direction 50A.
A skew correction process performed by the above-described structure will be explained below. When the sheet 505 is conveyed in an inclined orientation, the sheet 505 is conveyed in an inclined direction by the inclined roller 503. The sheet 505 conveyed in the inclined direction comes into contact with the wall that defines the reference position at an edge thereof, and the inclination of the sheet 505 is corrected as the sheet 505 is conveyed. Accordingly, the orientation of the sheet 505 is adjusted such that the edge of the sheet 505 extends along the wall. Since the reference surface 501 is provided to cause the sheet 505 to come into contact therewith, the reference surface 501 is hereafter called a contact surface 501.
After the orientation is adjusted, the sheet 505 is conveyed along the contact surface 501. Accordingly, the skew of the sheet 505 is corrected and the position of the sheet 505 in the width direction of the conveying path is determined. This structure is simpler and provides higher reliability compared to the case in which the paper feed accuracy is improved.
Two examples of known sheet-conveying devices will be described below with reference to published patent applications. As a first example, Japanese Patent Laid-Open No. 8-208075 discloses a sheet-conveying device including a contact surface fixed at a side of a sheet conveying path, a conveying roller, and an inclined roller, similar to the above-described structure. In this device, when it is detected by a sensor that a sheet is oriented straight after the sheet is caused to come into contact with the contact surface, the inclined roller is removed from the position where the inclined roller is in pressure contact with the sheet. Accordingly, the sheet can be conveyed without being influenced by the inclined roller.
As a second example, Japanese Patent Laid-Open No. 7-334630 discloses a sheet-conveying device that has a structure similar to the above-described structure but performs a characteristic operation before skew correction. More specifically, a plurality of sensors are provided above a sheet conveying path to detect the amount of inclination of a sheet, and the skew of the sheet is corrected by one of two methods depending on the detected amount of inclination. In a first method, unlike the above-described case, skew correction is performed while an inclined roller is separated from the sheet. More specifically, rollers are arranged at positions spaced from each other in a sheet-conveying direction and the inclination of the sheet is corrected by deflecting the sheet with the rollers such that the leading edge of the sheet comes into contact with a nip portion of the rollers. In a second method, the inclined roller is brought into pressure contact with the sheet and skew correction is performed using the inclined roller and a contact surface, similar to the above-described case.
In the above-described known devices, it is assumed that the sheet is conveyed in only one direction. In addition, whether or not to perform the skew correction is selected by either bringing the inclined roller into pressure contact with the sheet or separating the inclined roller from the sheet. In addition, the contact surface is fixed to the conveying path. For these reasons, the above-described known devices have the following problems.
That is, when, for example, the skew correction is not performed and the sheet is conveyed while the inclined roller is separated from the sheet, the width of the conveying path is limited since the contact surface is fixed to the conveying path. Therefore, when the sheet is conveyed in both forward and reverse directions as disclosed in Japanese Patent Laid-Open No. 2000-326531, the sheet comes into contact with the contact surface in the manner shown in FIG. 9. In addition, when the sheet is rotated on the conveying path as disclosed in Japanese Patent Laid-Open No. 5-213487, the sheet comes into contact with the contact surface in the manner shown in FIG. 10. Therefore, there is a risk that the contact surface 501 will obstruct the operations other than skew correction.
Accordingly, when the contact surface is fixed to the conveying path as described above, operations other than skew correction, such as two-way conveyance and rotation, cannot be performed.
Although the two-way conveyance and rotation of the sheet can be achieved when the width of the conveying path is increased or an additional conveying path is provided so that the sheet can be prevented from coming into contact with the contact surface, the size of the device is increased in such a case.
In addition, when the device is structured such that the inclined roller can move in the vertical direction so that the inclined roller can be separated from the conveying roller, the inclined roller must be supported in a movable manner while the contact surface is fixed to the conveying path. Therefore, as shown in FIG. 8, a supporting member 504 of the inclined roller 503′ is deflected when a reaction force is applied from the sheet. Accordingly, the position of the inclined roller 503 is shifted, which degrades the accuracy and reliability of the operation. In order to prevent this deflection, a material with high rigidity must be used.